The German scientist Walter Flemming was the first man to observe chromosomes while studying the dividing cells of salamander larvae in1882. In 1902 Walter Sutton, an American biologist gave supported evidence that Mendel's factors were found on these chromosomes. In 1909 Wilhelm Johannsen, a Danish biologist was the first to use the word gene instead of Mendel's word factor to describe the units of heredity. A gene is a portion of DNA that carries the information for producing a trait. After an experiment by Thomas Hunt Morgan in 1912, which proved Sutton's theory, biologists accepted the newly conceived chromosomal theory of inheritance. This theory states that genes on chromosomes are responsible for heredity.

Almost every cell in the human body has 46 chromosomes. All chromosomes can be easily sorted into 23 pairs. Each one has a match by shape and size, and are made up of genes that control the same trait. Biologists named cells which contain chromosomes in pairs as diploid, represented as 2n. A normal chromosome is made up of thousands of genes. They are made up of about 60 percent protein and 40 percent DNA.

There is only one type of cell that does not have 46 chromosomes and those are gametes, also called sex cells. The egg and sperm cells only contain 23 chromosomes and are called haploid, represented by n. These gametes have one chromosome for every pair. Gametes must have half the usual number of chromosomes so when the egg and sperm unite to form a new being it will have 46. Cell fusion is not a process that can continue to occur repeatedly. If it did continue, the amount of chromosomes in every cell would become enormous. Early biologists concluded that there must be a process during gamete production that would reduce the number of chromosomes to half the usual amount. Meiosis is the specific process by which reduction division occurs.